A number of different imaging technologies have been employed for recording images from digital data onto photosensitive media. Technologies adapted for digital printing onto photosensitive media include cathode ray tube (CRT), scanned laser beam, liquid crystal display (LCD) and digital micromirror device (DMD) technologies. Each of these imaging technologies achieves some measure of success for producing high quality prints competitive with more traditional optical imaging approaches.
In a CRT-based printer, digital image data modulates an electron beam, providing variable exposure energy as the beam scans across a phosphorescent screen. The emitted light is conditioned by color filters, then directed to the media through imaging optics. Examples of CRT-based systems include those disclosed in U.S. Pat. No. 4,754,334 (Kriz et al.) and in U.S. Pat. No. 5,303,056 (Constable).
In a laser-based printer, digital image data modulates the on-time duration or the intensity of a laser beam that is scanned across the media surface by a rotating reflector, such as a mirrored polygon. Relatively complex F-Theta optics are generally required for laser printing in order to minimize polygon-related artifacts and to provide the desired print resolution. One example of a laser-based printer is disclosed in U.S. Pat. No. 4,728,965 (Kessler et al.)
LCDs and DMDs are spatial light modulators that offer another alternative imaging solution. A spatial light modulator can be considered essentially as a one-dimensional (linear) or two-dimensional (area) array of light-valve elements, each element corresponding to an image pixel. In an LCD-based printer, digital image data modulates the polarization state of an incident optical beam. One example of an LCD-based printer is disclosed in U.S. Pat. No. 5,652,661 (Gallipeau et al.) In a DMD-based printer, digital image data modulates electrostatically deflectable mirrors, each corresponding to a single image pixel. One example of a DMD-based printer is disclosed in U.S. Pat. No. 5,461,411 (Florence et al.) Printing apparatus using either an LCD or a DMD spatial light modulator requires both illumination optics, to condition and direct incident light from a light source, and projection optics for directing modulated light onto the photosensitive medium.
While varying degrees of success have been achieved in deploying each of these technologies, some drawbacks remain. CRT imaging devices, for example, are bulky, limited in resolution, and relatively expensive. Laser scanning devices are characterized by relatively high cost and substantial optical and mechanical complexity. LCD-based printers require supporting optical systems for uniformization, polarization, illumination, and focusing of the modulated light. DMD printers also require costly support optics and are limited in the available resolution. Supporting optics for digital printing systems employing CRT, scanned laser, LCD, and DMD imaging devices add cost, size, and complexity to printing apparatus design. The significant size, complexity, and cost requirements prevent the building of compact digital printing systems, such as for handheld applications, and it can be seen that there is a need for a digital printing apparatus with simpler design and smaller form factor.
Photoemissive area arrays, such as the various types of organic light emitting diode (OLED) devices, have been developed chiefly for imaging as display devices. However, it has been recognized that there may be advantages to using OLED devices for printing onto photosensitive media. Photoemissive arrays can act as both light modulators and light emitters, and they can print a full image frame at a time. By eliminating the need of additional light source and illumination optics, photoemissive arrays can be employed to provide simplification to the design of printer systems. However, they still require separate projection optics for image forming on photosensitive media. Referring to FIG. 1, there is shown the basic optical path for a prior art printing apparatus 10 employing an conventional emissive array 19 with a lens 14 for imaging onto a photosensitive medium 16. Lens 14 could use a single element, as represented in FIG. 1, but would more likely include multiple optical elements, with chromatic correction, for example. The need for projection optics still poses limitations to the use of OLED devices in building very compact printer systems.
A notable configuration for printer design is contact printing, where an image is printed onto a photosensitive medium that is placed against or very near the exposing surface of a printhead. Without the use of interposed optics for imaging, contact printing has the advantage of allowing the implementation of potentially compact printer systems, but is not suitable for many digital printing technologies. In particular, light path constraints prevent laser-, LCD- and DMD-based imaging systems from being used in contact printing configuration. There has been disclosures of CRT-based digital printer designs utilizing contact printing. For example, as is disclosed in U.S. Pat. No. 4,484,233 (Strout et al.), CRT imaging is used in conjunction with a specially manufactured and treated fiber optic faceplate for forming an exposure image directly at the photosensitive media plane. Notably, the apparatus of U.S. Pat. No. 4,484,233 is sizable, requiring substantial depth for forming and modulating the imaging beam. Thus, although the apparatus of U.S. Pat. No. 4,484,233 does not require lens components for exposure of the photosensitive medium, it does require a sizable CRT system and a fiber optic faceplate that is specially designed and would be costly and not suitable for compact applications.
There is also an inherent device limitation that limits the suitability of OLED devices for use in contact printing designs. OLED devices are conventionally fabricated onto a glass substrate. Light emitted from the OLED material is dispersed as it travels through the transparent glass substrate. This causes a loss of sharpness and contrast, so that external supporting optics are generally unavoidable in order to use OLED devices in high resolution, high quality printing applications. Furthermore, OLED light emission is Lambertian, emitting equally in all directions. This is problematic for printing applications in that a significant fraction of the light from an OLED emitter at one pixel location exits the substrate at another pixel location, causing undesirable crosstalk. This further reduces sharpness and contrast for the image exposed on a photosensitive medium in contact printing configuration.
Thus, while the use of photoemissive arrays such as OLED offers the potential of achieving a compact printer design, inherent device limitations, as conventionally fabricated and deployed, prevent further simplification of printer design. There is, as yet, no clear solution of designing a high quality, photoemissive array printer without the cost, complexity, and space requirements of the optical subsystem between the printhead and the photosensitive medium. It can be seen that there is a need for improvement in a printing apparatus and method that achieves very compact printing at high image quality.